Design Patterns | Set 2 (Factory Method)
The factory method is a creational design pattern, i.e., related to object creation. In the Factory pattern, we create objects without exposing the creation logic to the client and the client uses the same common interface to create a new type of object.
The idea is to use a static member-function (static factory method) that creates & returns instances, hiding the details of class modules from the user.
A factory pattern is one of the core design principles to create an object, allowing clients to create objects of a library(explained below) in a way such that it doesn’t have a tight coupling with the class hierarchy of the library.
What is meant when we talk about libraries and clients?
A library is something that is provided by some third party that exposes some public APIs and clients make calls to those public APIs to complete their tasks. A very simple example can be different kinds of Views provided by Android OS.
Why factory pattern?
Let us understand it with an example:
I am two wheeler
What are the problems with the above design?
As you must have observed in the above example, the Client creates objects of either TwoWheeler or FourWheeler based on some input during constructing of its object.
Say, the library introduces a new class ThreeWheeler to incorporate three-wheeler vehicles also. What would happen? The client will end up chaining a new else if in the conditional ladder to create objects of ThreeWheeler. Which in turn will need the Client to be recompiled. So, each time a new change is made at the library side, the Client would need to make some corresponding changes at its end and recompile the code. Sounds bad? This is a very bad practice of design.
How do avoid the problem?
The answer is, to create a static (or factory) method. Let us see the below code.
I am three wheeler
In the above example, we have totally decoupled the selection of types for object creation from the Client. The library is now responsible to decide which object type to create based on an input. The client just needs to make calls to the library’s factory Create method and pass the type it wants without worrying about the actual implementation of the creation of objects.
Other examples of the Factory Method:
- Say, in a ‘Drawing’ system, depending on the user’s input, different pictures like squares, rectangles, the circle can be drawn. Here we can use the factory method to create instances depending on the user’s input. For adding a new type of shape, no need to change the client’s code.
- Another example: On the travel site, we can book train tickets as well as bus tickets and flight tickets. In this case, the user can give his travel type as ‘bus’, ‘train’, or ‘flight’.
Here we have an abstract class ‘AnyTravel’ with a static member function ‘GetObject’ which depending on the user’s travel type, will create & return an object of ‘BusTravel’ or ‘ TrainTravel’. ‘BusTravel’ or ‘ TrainTravel’ have common functions like passenger name, Origin, and destination parameters.
Further Read: Factory Method in Python
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